1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a printing method. In particular, the present invention relates to an ink jet printing apparatus and a printing method which print characters, images, etc. on printing media, such as printing paper and OHP paper, using an ink and a liquid (hereinafter referred to as a treating liquid) that insolubilizes the coloring material in the ink.
2. Related Background Art
The ink-jet printing method has various advantages including low noise operation, low running cost, high-speed printing, and easy adaptation for down-sizing and for color printing. Thus, ink-jet printing is widely employed for printers, copying machines, or the like. In general, the ink employed for printers is selected in consideration of its printing properties such as ejecting properties and fixing properties and print qualities such as bleeding, optical reflection density, and coloring of the printed images.
As is well known, inks are divided into two types according to the coloring material contained in the ink, i.e., the dye ink and the pigment ink. The pigment ink has various advantages in comparison with the dye ink, such as superior water resistance and light fastness, and clear character printing.
The pigment in a pigment ink is generally stably dispersed in the ink by the electric repulsion force of the polymer dispersant which destroys the agglomeration of the pigment particles due to intermolecular force. Therefore, it is preferable to add a polymer dispersant to an ink in an amount according to the amount of the pigment.
When such a pigment-based ink is applied onto a recording medium such as plain paper by an ink-jet recording process for printing characters, the solvent, e.g. water, of the ink penetrates into the paper and evaporates into the ambient air, and the pigment particles agglomerate. At that time, the more a polymer dispersant is added, the stronger the agglomeration force of the ink on the paper becomes. Thus, when an ink dot is formed on a paper sheet with a prescribed volume of the ink ejected from an ink-jet head, the diameter of the dot is small and the dot shape remains irregular due to the impact of landing on the paper. Therefore, in order to obtain an ink dot having a sufficient recording density and a diameter necessary for forming a recording image without white stripes or other similar defects, the ejection volume of an ink from an ink jet head must be adjusted rather high. Even with such an adjustment, the presence of a polymer dispersant in a pigment ink, in combination with the penetrability decrease of the ink due to the strong agglomeration force of the pigment particles adsorbed on the dispersant, may delay the fixation of a pigment ink on a recording medium or lower the rub-off resistance of the recorded image.
In order to make the dot diameter larger and to improve the fixation properties, a penetrant may be added to a pigment ink to enhance penetration of the ink into a recording medium. However, use of a penetrant may cause undesirable phenomena such as the irregular peripheral shape of the dot (feathering), and penetration of the ink to the back of the recording medium (back-through), which are undesirable for the high quality recorded image. Further, since the coloring material penetrates into the recording medium, it often occurs that the optical density (OD) of the ink dot is not so much increased as the dot diameter increases.
To solve such problems, inks containing a self-dispersing pigment have been proposed. Such an ink can provide dots of a larger diameter, since the pigment agglomeration force on the paper is weaker than that in conventional pigment inks in which the pigment is dispersed by a dispersant. It, however, is not sufficient yet.
As mentioned above, there are still much room for study and development in ink and printing method to satisfy various factors determining the quality of printing such as fixability of the ink, enlargement of the ink dot diameter, uniform density in an ink dot and high optical density of the ink itself.
On the other hand, in order to further improve the quality of printed characters and images, for example, to improve water fastness and optical density (OD) of the print, there has been proposed and practiced an ink jet printing method of providing an ink and a treating liquid reactive with the ink onto a printing medium to react them thereon.
In order to solve the above-mentioned problems of the pigment ink while maintaining the advantages of the pigment ink, the inventors of the present invention have been studying the above-mentioned ink-jet printing technology using a pigment-based ink and a treating liquid which breaks the dispersion state of the pigment in the ink by reaction. As a part of the study, the inventors have carried out a recording process, in which a pigment ink and a treating liquid are applied in this order or at the substantially same time onto the recording medium so as to be mixed each other in a liquid state. The quality of the resultant image was not necessarily satisfactory and in some cases the quality was rather inferior to that formed using the pigment ink only. Specifically, when a pigment ink containing a pigment dispersed in an aqueous medium by a polymer dispersant was used in combination with a treating liquid reactive with the pigment ink, sometimes OD reduction due to the low area factor of the obtained ink dot was observed. The reason why such a phenomenon occurs is not clear, but probably because the agglomeration of the pigment of the ink on the recording medium is promoted to a large extent by the treating liquid. Thus, to increase the optical density, the area factor may be increased by increasing the amount of the ink ejection, but sometimes leading to the inferior fixability.
Also, when a combination of a pigment ink containing a self-dispersing pigment and a treating liquid reactive with the ink is used, a phenomenon called xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is sometimes observed at the peripheral part of the ink dot formed on a recording medium interfering with clear dot formation. FIG. 1 is a schematic plan view of a dot (501) to which this oozing or haze phenomenon (502) has occurred. FIGS. 2A, 2B, and 2C illustrate the assumed mechanism of occurrence of this phenomenon.
When a treating liquid S is applied to a recording medium P (especially plain paper) after a pigment ink Ip containing a self-dispersing pigment but not containing a polymer dispersant was applied thereon as shown in FIGS. 2A and 2B, a reaction product 9 starts to be formed. As the reaction proceeds, radial xe2x80x9coozingxe2x80x9d occurs from an approximately circular dot of the reaction product as illustrated in FIG. 2C surrounding the dot with xe2x80x9chazexe2x80x9d. Such xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is recognized as the same as the known feathering in appearance, deteriorating the printing quality. In the combination of self-dispersion type pigment ink and a treating liquid S reactive with the pigment ink, a phenomenon 502 shown in FIG. 1, known as xe2x80x9cbleedxe2x80x9d or xe2x80x9chazexe2x80x9d, was sometimes observed in the resultant dot (refer to FIG. 1, 501) on a printing medium. FIGS. 2A, 2B and 2C are representations presumptively illustrating the mechanism of the occurrence of the above phenomenon.
When providing the treating liquid S onto the printing medium P (especially plain paper) on which the pigment ink Ip containing a self-dispersion type pigment but no macromolecular dispersant has been already provided (refer to FIG. 2A), the formation of a reaction product 503 begins (refer to FIG. 2B). And with the progress of this reaction, xe2x80x9cbleedxe2x80x9d of the reaction product begins to appear radially from the almost circular dot, as shown in FIG. 2C, and the dot as a whole looks as if a haze is hanging over it. This phenomenon, xe2x80x9cbleedxe2x80x9d or xe2x80x9chazexe2x80x9d, is apparently the same as the known phenomenon, feathering; therefore, it is the cause of the deterioration of printing quality.
Above described xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is assumed to be a chemical reaction or, in a micro-scale, a following phenomenon. A dispersant-free pigment ink reacts with the treating liquid at a relatively high reaction rate, so that immediately the dispersion break down of the dispersed pigment occurs to produce clusters of the reaction product. At the same time, fine particles of the reaction product are also produced, which then flows out into the recording medium with the penetrating treating liquid to cause xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d.
As described above, mere combination of a pigment ink and a treating liquid would cause unexpected phenomena, making it difficult to obtain high quality ink-jet recording images. The inventors has recognized the necessity of the further technical development of an ink-jet recording technology utilizing a treating liquid, overcoming the disadvantages and retaining the advantages of the pigment ink.
The present invention is achieved on the basis of newly obtained technical knowledge and findings. Objects of the present invention are to provide an ink-jet recording apparatus and an ink-jet printing method for obtaining a high quality print, using a treating liquid and a pigment ink in ink-jet printing.
According to one aspect of the present invention, there is provided an ink jet recording apparatus which comprises an ink ejection portion for providing onto a printing medium an ink containing a pigment dispersed in an aqueous medium, a treating liquid ejection portion for providing a treating liquid reactive with the ink: and control means for controlling the ink ejection portion and the treating liquid ejection portion so as to provide onto the printing medium the ink and the treating liquid in this order,
wherein the ink contains an aqueous medium, a first pigment, a second pigment and a polymer dispersant, the first pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group, the second pigment being a pigment which can be dispersed in the water-base medium with the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer,
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained In the ink when the treating liquid and the ink are mixed on the printing medium in a liquid state, and
the control means controls the ink ejection portion and the treating liquid ejection portion so that the ink and the treating liquid are provided onto the printing medium respectively from respective ejection portions and mixed with each other in a liquid state on the printing medium.
According to another aspect of the present invention, there is provided an ink jet recording apparatus which comprises an ink ejection portion for providing onto a printing medium an ink containing a pigment dispersed in an aqueous medium, a treating liquid ejection portion for providing a treating liquid reactive with the ink; and control means for controlling the ink ejection portion and the treating liquid ejection portion so that the ink, the treating liquid and the ink are provided onto the printing medium in this order,
wherein the ink contains an aqueous medium, a first pigment, a second pigment and a polymer dispersant, the first pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group, the second pigment being a pigment which can be dispersed in the aqueous medium with the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer,
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained in the ink when the treating liquid and the ink are mixed on the printing medium in a liquid state, and
the control means controls the ink ejection portion and the treating liquid ejection portion so that the ink and the treating liquid are provided onto the printing medium respectively from respective ejection portions and mixed with each other in a liquid state on the printing medium, and the control means further controls the ink ejection portion to mix the ink into the liquid mixture of the ink and the treating liquid.
According to a still further aspect of the present invention, there is provided an ink jet printing apparatus which comprises an ink ejection portion for providing onto a printing medium an ink containing a pigment dispersed in an aqueous medium, a treating liquid ejection portion for providing a treating liquid reactive with the ink; and control means for providing the printing medium the ink and the treating liquid in this order,
wherein the ink contains an aqueous medium, a first pigment, a second pigment and a polymer dispersant, the first pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group, the second pigment being a pigment which can be dispersed in the aqueous medium with the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer,
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained in the ink when the treating liquid and the ink are mixed on the printing medium in a liquid state,
the ink ejection portion includes at least one pigment ink ejection portion for discharging the ink, and
the control means comprises an arrangement means for arranging the pigment ink ejection portion and the treating liquid ejection portion in a predetermined position; ejection control means for controlling relative position of each ejection portion and controlling the ejection portions so as to eject the ink and the treating liquid from the respective ejection portions and mix them with each other on the printing medium.
According to a still further aspect of the present invention, there is provided an ink jet printing apparatus which comprises an ink ejection portion for providing onto a printing medium an ink containing a pigment dispersed in an aqueous medium, a treating liquid ejection portion for providing a treating liquid reactive with the ink; and control means for controlling the ink election portion and the treating liquid ejection portion so as to provide onto the printing medium the ink and the treating liquid in this order,
wherein the ink contains a water-base medium, a first pigment, a second pigment and a polymer dispersant, the first pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group, the second pigment being a pigment which can be dispersed in the aqueous medium by the use of the polymer dispersant, the polymer dispersant comprising at least a nonionic polymer and an anionic dispersant with the same polarity as that of the group bound to the surface of the first pigment,
the treating liquid contains at least one multivalent metal cation selected from the group consisting of Ca++, Cu++, Ni++, Mg++, Zn+++, Ba++, Al+++, Fe+++, Cr+++, Co++, Fe++, La++, Nd+++ and Y+++ or the salt thereof, or both of the multivalent metal cation and the salt thereof, and
the control means controls the ink ejection portion and the treating liquid ejection portion so that the ink and the treating liquid are provided onto the printing medium separately from respective ejection portions and mixed with each other in a liquid state on the printing medium.
According to a still further aspect of the present invention, there is provided an ink jet printing apparatus which comprises an ink ejection portion for providing onto a printing medium an ink containing a pigment dispersed in an aqueous medium, a treating liquid ejection portion for providing a treating liquid reactive with the ink; and control means for controlling the ink ejection portion and the treating liquid ejection portion so as to provide onto the printing medium the ink and the treating liquid in this order,
wherein the ink comprises a first ink containing a self-dispersing pigment in an aqueous medium, the self-dispersing pigment having at least one anionic group attached on the surface thereof directly or via other atomic group, and a second ink containing a second pigment and a polymer dispersant, the second pigment being a pigment which can be dispersed in the water-base medium by the use of the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer,
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained in the ink when the treating liquid and the ink are mixed on the printing medium in a liquid state, and
the control means controls the ink ejection portion and the treating liquid ejection portion so that the ink and the treating liquid are provided onto the printing medium separately from respective ejection portions and mixed with each other in a liquid state on the printing medium.
According to a still further aspect of the present invention, there is provided an ink jet printing method for forming an image on a printing medium which comprises the steps of:
(i) applying an ink on a printing medium by ink jet method; and
(ii) applying a treating liquid reactive with the ink on the printing medium,
wherein the ink contains a water-base medium, a first pigment, a second pigment and a polymer dispersant, the first pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group, the second pigment being a pigment which can be dispersed in the aqueous medium with the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer,
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained in the ink when the treating liquid and the ink are mixed on the printing medium in the liquid state, and
the step (ii) is carried out subsequently after the step (i) or substantially at the same time in such a manner that the ink and the treating liquid come to contact in a liquid state on the printing medium.
According to a still further aspect of the present invention, there is provided an ink jet printing method which comprises the steps of: applying a first ink, applying a second ink and applying a treating liquid, in such a manner that the inks and treating liquid come into contact with each other in a liquid state on a printing medium,
wherein the first ink contains a self-dispersing pigment in an aqueous medium, the self-dispersing pigment being a self-dispersing pigment having at least one anionic group bound to the surface thereof directly or via other atomic group,
the second ink contains a second pigment and a polymer dispersant, the second pigment being a pigment which can be dispersed in the water-base medium with the polymer dispersant, the polymer dispersant comprising at least one of a nonionic polymer and an anionic polymer, and
the treating liquid contains at least one of a multivalent metal cation and a salt thereof, that aggregates at least either of the pigments contained in the ink when the treating liquid and the ink are mixed on the printing medium in a liquid state, and
the treating liquid is applied to the printing medium after at least either of the first ink and the second ink is applied onto the recording medium.
By the invention according to respective embodiments as described, high quality images of high OD and high edge sharpness can be obtained and various advantages such as rub-off resistance and enhanced fixability can be obtained.
It is still not clear the reason why such advantages are obtained by providing a treating liquid containing multivalent metal ion or the salt thereof subsequently or substantially at the same time as providing ink containing first and second ink produces such effects. However, through various experiments concerning the present invention, the present inventors have confirmed the following facts. When an ink containing the above first and second pigments is provided onto a printing medium, an ink dot of a certain size is formed on the printing medium P as shown in FIG. 3A, of which size (diameter: d1) is larger than that (diameter: d2, FIG.3B) formed with a conventional pigment ink, such as an ink containing a pigment dispersed by the polymer dispersant or an ink containing a self-dispersion type pigment (d1 greater than d2). The reason is not still clear. Presumable mechanism is as follows: The second pigment on which a macromolecular dispersant is adsorbed and the self-dispersing first pigment electrically repel each other in the ink, and the aggregation force of the pigments is weaker than that in an ink in which the pigment is dispersed by a polymer dispersant. When the latter ink is printed on a recording medium, the second pigment having a macromolecular dispersant adsorbed thereon would not diffuse rapidly in the direction perpendicular to its surface (in the longitudinal direction). The diffusion in the direction along the paper surface (in the transverse direction) is also difficult, since the polymer dispersant molecules rapidly intertwine with each other or cross-link the pigment molecules as the water content decreases due to the evaporation and penetration into the recording medium, to cause strong aggregation of pigment. On the other hand, with the ink according to the present invention, the existence of the first pigment prevents or inhibits the macromolecular intertwining or cross-linking of pigments. In addition, the repellency between the first pigment and the polymer dispersant eases the strong intermolecular force between pigment molecules. As a result, the ink is liable to diffuse in the transverse direction. The diffusion is eased but not in disorder, since it is still under the influence of pigment aggregation force.
When the treating liquid S, which contains multivalent metal ion or the salt thereof, onto an ink dot diffusing into the recording medium uniformly and widely as described above (FIGS. 2B and 2C), a reaction (ionic reaction between the pigment and the multivalent metal ion or salting-out) occurs at the interface between the ink and the treating liquid, leading the first and second pigments stably dispersed in the ink to separate out. This reaction is considered to complete within a very short time, since the height (t1) of the ink dot is lower than that (t2) of the conventional ink dot because the ink diffuses wider than the conventional ink described above. Presumably, the above mechanism reduces the fixing time and improves the fixing characteristics, and moreover, improves the edge sharpness of the ink dot in the embodiments of the present invention. It will be understood that the effects of the present invention due to such a mechanism are peculiar to the system in which the ink and the treating liquid are provided onto a printing medium in this order or substantially at the same time.
Further, use of a treating liquid having high penetrability to a printing medium in the present invention improves fixing characteristics and edge sharpness of an ink dot much more. This may be because the high penetrability of the treating liquid accelerates the penetration of the ink solvent including water, while it is reacting with the ink on the surface of the printing medium. Thus, the solvent penetrates deeper into the printing medium. In general, the penetration of the coloring material into the printing medium tends to lower the optical density, but when an ink is provided onto a printing medium before a treating liquid as in the present invention, it is rare for the pigment to penetrate into the printing medium so deep as to decrease OD. On the contrary, it has been found that the coloring material tends to remain on or near the surface of the printing medium because of the reaction with the treating liquid. As a result, OD is improved compared with the case where no treating liquid is used.
Further, higher image quality can be achieved by optimizing the species and concentration of the multivalent metal ion or the salt thereof used in the treating liquid according to the types and ratio of the first and second pigments.
In the embodiments of the present invention, it is preferable to change the application amount of the treating liquid according to the species and ratio of the first and second pigments in the ink, in order to achieve higher image quality.
Thus, it becomes possible to form images of very high quality, i.e, high OD, free from xe2x80x9chazexe2x80x9d and excellent fixation, within a very short fixing period.